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Testing for Number Arguments
Resources
- isNaN() – MDN
- 3 Ways to Check if Variable is a Number in JavaScript
- The throw statement – MDN
- Error objects – MDN
- First-class Function – MDN
Functions as First-class Citizens
Functions are often referred to as "first-class citizens" in JavaScript, which means that you have the ability to do almost anything with functions.
You can store a function in a variable:
const multiply = function(a, b) {
return a * b;
};
Pass a function as an argument to other functions:
function sayYay() {
return 'Yay';
}
/*
- The argument passed for 'func' should be a reference to a function
- That function gets called in console.log
*/
function greeting(func, name) {
console.log( `${func()}, ${name}!` );
}
/*
- A reference to the sayYay function gets passed to the greeting function as an argument
- sayYay returns the string 'Yay', so the greeting function logs 'Yay, Lee!'
*/
greeting(sayYay, 'Lee'); // Yay, Lee!
You can even set a function to return another function:
function sayYay() {
return function() {
console.log('Yay!');
}
}
This lets you take advantage of powerful design patterns you'll learn about in later courses.
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Now we need to make sure that
the getRandomNumber function
0:00
receives two numeric values as arguments.
0:03
And no other type of values.
0:06
If the function is called
with a string for example,
0:08
then the program should display
a custom error message.
0:11
Inside the function add an if statement
just above the return statement.
0:14
But what should we test here?
0:19
Well, we need to make sure that the two
arguments passed to the function
0:22
are numbers.
0:25
There's a few ways we might do this.
0:26
I’ll teach you a new method for
testing number values.
0:28
JavaScript provides a special function
called isNaN, or is not number.
0:31
That takes one argument and
returns a Boolean value.
0:36
It returns true if the value is
not a number, and false if it is.
0:39
For example, if you pass the string 'six'
to the function, you get the value true.
0:44
However if you pass a number, like six,
to the function, you get false.
0:49
So in the if statement,
I'll first test that the lower argument
0:54
passed to the function is a number,
like this.
0:58
In other words,
if the lower value is not a number,
1:04
then the code block with
this if statement will run.
1:07
But we want to test both arguments.
1:11
If either is not a number,
then we'll display an error.
1:13
To test both you can use the logical or
operator, by typing two pipe characters.
1:17
Then add a test for
the upper argument like this.
1:22
So now if either argument is not a number,
we can display a custom error.
1:30
We could maybe use console.log to
display a message in the console.
1:36
Like 'Both arguments must be numbers.'.
1:41
In this case, it's probably
best to throw a custom error.
1:50
So the programmer can immediately see that
they've used the function incorrectly.
1:53
JavaScript lets you create your own errors
which print directly to the JavaScript
1:57
console, just like
regular syntax errors do.
2:01
It looks like this.
2:04
The throw keyword,
followed by JavaScript's Error object.
2:06
This is a built in keyword and
2:12
object JavaScript provides
to create a custom error.
2:14
Or, as developers say, throw an exception.
2:17
Since it's inside the if statement,
the error is thrown
2:20
only when the function receives anything
but a number for either argument.
2:24
All right, now I'll test the latest
changes by calling the function correctly
2:29
with numbers a couple of times.
2:33
Then call it with a string
value like 'three hundred'.
2:37
I'll save the file.
2:46
And over in the console,
I get two random numbers, 4 and 17.
2:47
And below them is the Uncaught Error,
displaying my custom error message,
2:52
Both arguments must be numbers.
2:56
Notice how it looks just like a regular
error thrown by the JavaScript engine.
2:59
It even displays the line number
producing the error, great.
3:02
So now you've learned how
to program defensively.
3:06
Or looking out for problems that might
arise in your program and taking action.
3:09
As with many things in programming,
3:14
getting comfortable with the details
of functions takes time and practice.
3:16
But once you get to hang of working with
functions, you'll be well on your way to
3:19
mastering one the most essential
features of JavaScript.
3:23
In fact, functions are often referred to
as first-class citizens in JavaScript.
3:26
Which means that you have the ability
to do almost anything with functions.
3:31
Like store them in variables,
pass them as arguments to other functions.
3:35
You can even set a function
to return another function.
3:39
This lets you take advantage of powerful
design patterns you'll learn about in
3:41
later courses.
3:45
So you should feel excited and
3:46
proud of reaching another important
milestone in your programming journey.
3:47
And remember, we're here to help.
3:51
So if you have questions about anything
covered in this course, you can always get
3:53
in touch with the friendly Treehouse
staff or other students in the community.
3:56
Thanks everyone, and happy coding.
3:59
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